The present invention is directed to pump cavities for lasers. It specifically concerns such cavities for laser systems in which the laser rod is to be pumped by a plurality of lamps or a plurality of rods are to be pumped by a single lamp.
The power for an optically pumped solid-state laser is typically provided by one or more flash lamps exterior to a rod of laser material. The lamps shine on the rod, and the radiation promotes electrons in the laser rod to higher energy levels, from which they are stimulated to fall by lasing action to cause coherent laser light.
In high-power lasers, it is important that the radiation from the flash lamps be converted efficiently to electron energy. This requires that some of the radiation from each the lamp be reflected so that it reaches the laser rod rather than radiating off into space. However, any reflection results in some power loss, so it is desirable to keep the number of reflections in the paths from the lamp to the rod as low as possible. It is also desirable to prevent the radiation from being reflected back into the flashlamp, since such reflections also result in a loss of efficiency.
It has been shown that the best uniformity of radiation, and the theoretically highest etendue, i.e., the highest throughput or efficiency of conversion of lamp-radiation energy to electron energy, are achieved if a reflector surface is used that touches the surfaces of the laser rod and lamp and forms curves similar to those shown in FIG. 1. The perimeter of the radiating surface of the lamp 14 is equal to the perimeter of the laser rod 12, and the reflector curves are generated in a "constant-string-length" method similar to one that will be discussed below.
FIG. 1 shows the cross section of a laser rod 12 and a flash lamp 14 enclosed by two reflecting walls 16 and 18 that contact the laser rod 12 and the lamp 14. In practice, however, it ordinarily is not possible to bring the reflecting surface completely into contact with the radiating surface of the lamp, and it may also be impractical to bring it into contact with the rod surface, so it is necessary to settle for an efficiency lower than the theoretically possible level.
The highest efficiency available, given the requirement of particular separations of the reflector wall from the lamp and the rod, can also be generated by a "constant-string-length" method. This method as used for a particular pair of separations is illustrated in FIG. 2. It will be assumed that the reflecting wall can be brought no closer to the rod surface than d.sub.1 and no closer to the lamp surface than d.sub.2. These distances thus define circles 20 and 22 from the interior of which the reflecting wall must be excluded.
In FIG. 2, two tangent lines 26 and 28 intersect a base line 29 defined by the axes of the lamp and rod. Tangent lines 26 and 28 define points of tangency 30 and 32 and 34 and 36, respectively. The curve that results in the most efficient and uniform transfer of power can be thought of as being generated by a pencil 38 whose tip holds taut a string 40 secured to the rod and lamp at tangent points 32 and 34. The pencil 38 begins at a so-called cusp point 42. Cusp point 42 is the point colinear with the lamp and rod axes, disposed on the side of the lamp 14 opposite the rod 12, and spaced by the lamp stand-off distance d.sub.2 from the surface of the lamp 14. With the initial length of the string being such as to render it taut when the pencil is disposed at cusp 42, the pencil draws curve 44 while keeping string 40 taut. Eventually, the curve terminates on a rod cusp 46.
A lower curve is generated similarly, but with the string attached to tangent points 30 and 36 rather than tangent points 32 and 34.
The curves 16 and 18 of FIG. 1 are generated similarly, the most apparent difference being that the cusps lie on the surface of the lamp 14 and the rod 12. Another difference between the arrangements of FIGS. 1 and 2 is that, while the actual perimeters of the laser rod 12 and the lamp 14 are equal in the FIG. 1 arrangement, they may be different in the FIG. 2 arrangement, because the relationship of the lamp and rod sizes is such that the "effective perimeters" are equal. The effective perimeter for the laser rod 12 is the shortest distance from its cusp 46 around the rod 12 and back to the cusp 46. The equivalent perimeter for the lamp 14 is similarly the shortest distance from its cusp 42 around the lamp 14 and back to its cusp 42. If the effective rod and lamp perimeters are equal, the actual lamp and rod perimeters differ when the lamp and rod stand-off distances are different.
With the arrangement of FIG. 1, there are points on the lamp 14 from which light strikes the laser rod 12 either directly or after only a single reflection from surface 16 or 18, depending on the angle from which the light leaves the point. For other points, namely, those for which the laser rod 12 is beyond the "horizon," the light hits the rod 12 after one or two reflections, the number of reflections again depending on the angle at which the light leaves the point. If the total radiation produced by the lamp is considered, the average number of reflections is as low as is theoretically possible, and none of the light is reflected back to the lamp. Thus, the etendue, or throughput, is at a theoretical maximum, as is the radiation uniformity. Uniformity is important because the rod size needed for a given power level is smallest if the radiation that it receives is uniform.
With the FIG. 2 arrangement, the reflecting surface does not touch the surfaces of the lamp 14 and rod 12, so some of the light is reflected back into the lamp 14, and the throughput is less than the theoretical maximum. However, it is the maximum uniform throughput that can be achieved with reflecting surfaces that are required to stand off by the given distances.
While the foregoing designs result in a level of throughput and pumping uniformity that are the highest possible in view of the stated design constraints, their application is limited. The laser rods in some applications must have large diameters in order to keep the laser-radiation fluences at levels below the damage threshold of the laser material. In the foregoing designs, it is necessary that the circumference of the flash lamp roughly equal that of the laser rod. This requirement is often at odds with optimum flash-lamp design parameters. Accordingly, multiple flash lamps are sometimes used. But multiple-lamp designs have in the past departed from the optimum throughput and uniformity provided by the foregoing designs.
It is accordingly an object of the present invention to provide multiple-lamp systems with the throughput and uniformity of the single-lamp designs described above.